1. Field of the Invention
The present invention relates to an SOI substrate and a method of producing the SOI substrate having an insulation layer in a single crystal silicon substrate.
2. Description of the Related Art
It is known well that the thin single crystal silicon layer provided on an insulation layer is preferable than making an integrated circuit on the bulky single crystal silicon substrate in view of the electrical characteristics of the elements and separation of the respective elements.
A semiconductor substrate having, in an electrically insulated state, the surface single crystal silicon layer, in which the elements are formed, by making an insulation layer (SiO.sub.2) in the single crystal silicon substrate is generally called as "SOI (Silicon On Insulator)". The semiconductor substrate can be produced by a wafer bonding method or an oxygen-ion implantation method. The former wafer bonding method is that an oxide layer is formed on the single crystal silicon substrate, next an other single crystal silicon substrate is stuck to join thereon and a thin active silicon layer is formed by means of the whetted one of the single crystal silicon substrate. The later oxygen-ion implantation method is called as SIMOX (Separation by IMplanted OXygen) and has attracted attention lately to be preferable to form a thin-film active silicon layer to avail of a large-scale integratize, which forms a buried oxide layer inside of the silicon substrate by being allowed to react with Si and O in a high temperature anneal processing (1,100-1,200.degree. C.) after implanting a high-dose oxygen ion (.sup.16 O.sup.+) into the single crystal silicon substrate. Particularly, a SIMOX substrate can cause the single crystal silicon substrate to be an active layer having an uniform thickness, though the surface single crystal silicon layer is not received with a processing of whet as a sticky substrate.
However, when the buried oxide layer is formed in the above-stated silicon substrate by the anneal processing after implanting the oxygen ions into the single crystal silicon substrate, there are following disadvantages.
(1) The high quality SIMOX substrate has a low dislocation density of the surface single crystal silicon and surpasses in the electrically insulated state of the buried oxide layer. But, since the buried oxide layer is thin and the thickness is 80-90 nm, if a particle 10 attaches on a surface of a single crystal silicon substrate 1 when the oxygen ion is implanted as shown in FIG. 9, impossible parts of being implanted will be remained in a high-dose oxygen-ions implantation layer 3 because the particle 10 takes place a masking effect. The high-dose oxygen-ions implantation layer 3 resultes in a buried oxide layer 5 by means of the anneal processing, but since the afore-mentioned impossible part of being implanted with the oxygen ions resultes in a pinhole 9, an electric insulation property is inferior. Incidentally, 2 means the surface single crystal silicon layer and 6 means a surface oxide layer formed by the anneal processing in FIG. 9.
(2) FIG. 10 charts a correlation between the dose of the oxygen ions and the dislocation density in the surface single crystal silicon layer (see J. Mater. Res., Vol. 8, No. 3, March 1993 pp. 523-534). In a substrate increased in a thickness of the buried oxide layer by being increased in the dose of the oxygen ions to 1.0.times.10.sup.18 /cm.sup.2 -2.0.times.10.sup.18 /cm.sup.2, a crystal defect, or the dislocation density is increased in the surface of single crystal silicon layer. Particularly, in and over 1.5.times.10.sup.18 /cm.sup.2, the dislocation density is suddenly increased.
(3) In the substrate increased in the dose of the oxygen ions to 0.5.times.10.sup.18 /cm.sup.2 -0.9.times.10.sup.18 /cm.sup.2 within the range of the non-increasing crystal defect density, a strength of breakdown electric field in the buried oxide layer is a small range of 0-1 MV/cm and the electric insulation property is low as charted in FIG. 11.
(4) In the SIMOX substrate having the thin buried oxide layer, irregularity of an interface of the surface single crystal silicon and the buried oxide layer (abbreviated to "an buried oxide layer interface") is large (square root mean roughness Rms is about over 2 nm), a electrical characterstics of devices varies widely in ease. The thus SIMOX substrate can be sufficiently practiced to be flatness of the buried oxide layer through spending much time on the anneal processing, but it is impossible in view of its cost.
Accordingly, it is the first object of the present invention to provide an SOI substrate which can increase a thickness of its buried oxide layer without increasing the dose of implanted oxygen ions to prevent an increase of crystal defect in a surface single-crystal silicon layer. The second object of the present invention is to provid a method of producing the SOI substrate capable of decreasing a pinhole density in the buried oxide layer. The third object of the present invention is to provide a method of producing the SOI substrate which improves the flatness of the buried oxide layer interface. It is also an object of the present invention to provide the SOI substrate in which the crystal-defect density of the surface silicon layer is little, the buried oxide layer is increased more than a theoretical buried oxide layer thickness calculated from the dose of the implanted oxygen ions, the generation of pinholes is very little, and the flatness of the buried oxide layer interface is improved.